Design Considerations and Best Practices for Tank Vent Filtration

Understanding the diverse applications of vent filters is critical to their proper implementation and use.

By Michael Felo, EMD Millipore

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Many biopharmaceutical applications require vent filters—hydrophobic sterilizing-grade filters used as air vents on processing tanks. The purpose of the tank vent filter is twofold:  maintain near ambient pressure in the tank while ensuring sterility in the tank. The tank vent filter removes viruses and microorganisms from the gas as it flows into or out of the tank.

To ensure proper operation and sterility, a bioreactor, for example, may have a number of vent filters including those for the tank vent, the sparge gas inlet, and the overlay gas inlet. 

Understanding the diverse applications of vent filters is critical to their proper implementation and use.
A number of factors must be considered in advance of vent filter implementation including filter sizing, housing and piping design, condensate control, regulatory requirements, and operational considerations such as clean- and steam-in-place (CIP, SIP) and integrity testing. Employing best practices for vent filter use can avoid common problems during installation, CIP, SIP, integrity testing, and operation.

This article will describe an application-based approach to vent filter sizing, in situ integrity testing of the filter design, best practices for housing design and vent filter operation, and a risk management approach to implementation and replacement of vent filters. 

Vent Filter Sizing

Air flows in and out of a process tank commonly for two reasons: the first is to replace a volume of liquid as it is pumped in or out of the tank. Sizing the tank vent filter for pump-out or fill rate is relatively simple as the air flow rate will be equal to the pump-out or fill rate. With flow rate and pressure determined, a flow/pressure change (ÄP) curve can be used to determine sizing.

The second reason air will flow into a process tank is to compensate for the volume change associated with steam condensation. At the end of a tank SIP procedure, steam in the tank will cool and undergo a phase change to liquid water. There is more than a 1,000x difference in volume between an amount of water in gas phase vs. liquid phase. During cooling, sterilized ambient air must be allowed into the tank to prevent vacuum. Sizing the vent filter for steam collapse requires knowing the vacuum rating of the tank and the convective cooling rate. These can be calculated based on the tank dimensions including height, diameter, and wall thickness. At EMD Millipore, we have developed a computer program to facilitate these calculations.

Improper tank vent sizing can result in low pump-out rates, loss of sterility due to a ruptured disk or filter failure, or worst case, tank implosion. Fortunately, proper sizing is not difficult as long as the flow requirements and driving force are understood.

Tank venting can be static or dynamic with each requiring filters that are sized slightly differently. For static venting, the air outside of the tank is assumed to be at ambient pressure, so the driving force for airflow is determine by the pressure difference between the inside of the tank and the atmosphere. For dynamic venting, compressed air is fed to the tank in order to minimize any difference in pressure that occurs between the tank and atmosphere.

Static tank venting is commonly used for buffer tanks and intermediate storage tanks. To determine the proper size for a static tank vent filter, we utilize a four-step process:

  1. Determine the maximum flow rate for venting that the vent filter will need to provide. This will be either the process flow rate or steam collapse rate after an SIP cycle.
  2. Select the maximum pressure drop that you want the filter to experience.  The pressure drop is typically less than 5 psi and should be dictated by the vacuum rating for the tank or rupture disk vacuum rating. Clearly, it is important to avoid pulling a strong vacuum on the tank that might cause collapse.
  3. Calculate the number of filters or filter area required to meet flow rate and pressure drop requirements.
  4. Ensure an adequate safety factor (~1.5x) and select the appropriate filter configuration for the tank or application.

Table 1 shows the vent sizing process for a static vent. This example represents a hold tank used for ambient temperature water storage with a maximum pump-out rate of 400 liters per minute.  No SIP is necessary and the tank has no vacuum rating. 
Pharma filtration   
    Table 1. Vent sizing process for a static vent.

Common uses for dynamic tank venting include bioreactors and other applications where steam is replaced with compressed air after an SIP cycle. In this case, the process for sizing a filter varies slightly from the static model and is as follows:

  1. Select the desired pressure drop. Pressure drop is typically less than or equal to 2 psi, especially when calculating for bioreactors where minimizing vacuum is key to maintaining a sterile environment in the tank.
  2. Calculate the air flow rate necessary to replace the steam during steam collapse post-SIP.
  3. Calculate the number of filters or filter area needed to meet flow rate and pressure drop requirements.
  4. Ensure an adequate safety factor (~1.5x) and select the appropriate filter configuration for the tank or application. 

Dynamic vent sizing can be significantly more complex than static venting since the steam collapse rate needs to be calculated for the specific tank and process conditions being used. In this case, it is recommended that software, such as that developed by EMD Millipore, be used to calculate the proper dynamic vent filter sizing.

In Situ Integrity Testing of Filters

Vent filters are typically tested using the Hydrocorr water flow integrity test (Hydrocorr Validation Guide, MM document VG050).1 The test measures the resistance of the filter to water intrusion (Figure 1). A filter is placed in a stainless steel housing which is flooded with water. Under a pressure of 40 psi, there is compaction of the filter. Over time, the compaction stabilizes and the flow decreases.  Once stabilization is complete, an instantaneous flow measurement can be taken; if the measurement is below the specification of the vent filter, it is considered to be integral. 

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